The mammalian target of rapamycin (mTOR) plays a fundamental role in the regulation of skeletal muscle mass, however, the molecular mechanism(s) that regulate mTOR signaling have only been vaguely defined. Nevertheless, advances are being made, and recent studies suggest that the highly conserved translationally- controlled tumor protein (TCTP) can regulate mTOR signaling by acting as a guanine exchange factor (GEF) for the GTPase Rheb. Furthermore, previous studies have shown that the expression of TCTP can be rapidly induced by growth promoting stimuli through a mechanism that is sensitive to the mTOR inhibitor, rapamycin. Based on these observations, we have envisioned the potential for a mechanism in which TCTP regulates mTOR signaling in a feed-forward manner, and such a mechanism might explain how an acute growth stimulus (e.g. 60 high-resistance contractions) can promote a highly prolonged (>36hr) activation of mTOR signaling. In support of this possibility, we have found that TCTP expression is elevated in response to mechanical loading via an mTOR-dependent mechanism. Furthermore, we have determined that overexpression of TCTP is sufficient to induce hypertrophy. Combined, these observations have led us to our central hypothesis: growth promoting stimuli induce TCTP expression via an mTOR-dependent mechanism, and changes in TCTP expression, in-turn, regulate mTOR signaling, protein synthesis and fiber size. To test this hypothesis we will use a combination of in vivo biochemical, molecular and genetic approaches while pursuing the following three specific aims: 1) Determine if various growth promoting stimuli induce TCTP expression via an mTOR- dependent mechanism; 2) Determine if the expression of TCTP regulates protein synthesis and fiber size; and 3) Determine if TCTP induces hypertrophy through an mTOR-dependent mechanism that requires TCTP's GEF activity towards Rheb. The results of these studies are expected to firmly establish TCTP as novel regulator of skeletal muscle mass and they will provide insight into how TCTP exerts this effect. Furthermore, if the central hypothesis is correct, the outcomes will expose a mechanism that substantially advances our understanding of how mTOR signaling and skeletal muscle mass are regulated.